Square punches for making recessed fasteners have many defects. One particular problem is in making the punch itself, which, because of its square configuration, is not easily formed. Another problem is to punch a square recess into a screw head without distorting the head out of true circular form. Still another problem is that the amount of metal working required in puching a square recess by a single blow into a circular blank head at room temperature is very considerable and is liable to create non-uniform stresses in the head. This is particularly true where the screw is to be formed of stainless steel or other hard to work with metals. At the same time the number of critical wear absorbing surfaces on square punches are limited, such wear occurring particularly at the four bottom corners of the punch; with the result that punch wear is quite considerable.
Also in the punching of a square recess into a round blank there is a tendency of the metal in the screw head to fall away from the punch if the side walls of the socket are made parallel to the screw axis and accordingly the socket walls must be inclined to such axis. Because of this side wall taper it has been found that any appreciable wear on the punch will provide a recess formation that will have a different bottom dimension than the recess originally produced by the punch and the appropriate driver designed for the original recess will not seat fully home and will not be in optimum fit with the recess. As a result punch life is a significant factor in limiting production and manufacturing costs, and it would be of very considerable importance to providing a "square" socket formation while extending punch life.
Prior art punches having flat walls at right angles to each other are easily broken by side deflection forces which occur when the punch and blank are not accurately aligned. Further, metal does flow evenly over the flat walls of square punches, and this results in numerous defects in the end product.